The present invention relates to a method of charging a secondary battery, a method of calculating a remaining capacity rate of the secondary battery, and a battery pack.
Presently, for many of apparatuses used with batteries, a rechargeable secondary battery is used and charged with a charging apparatus at user's home and the like. These apparatuses are so configured as to indicate, by color or blinking of a lamp or a liquid crystal display manner, and the like, that a charge is in progress, or the charge is completed, or alternatively, to give an indication of a battery capacity for permitting a user to know a rough remaining usable time.
Methods of detecting a charging capacity and a remaining capacity of the secondary battery may include an integrating method in which the charging capacity is calculated by integrating a current or a power output and a voltage method in which determination is performed whether or not a full charge has been reached by measuring a battery voltage.
As described the above, the integrating method is of integrating the current or the power output, and is thus capable of detecting the absolute charging capacity without any influence of voltage fluctuation, thereby making detection of the charging capacity easy. Further, as shown in FIG. 1, a charging rate (the charging. capacity) increases linearly in an early period of the charging process, so that calculation of the charging rate may be performed accurately.
On the other hand, in the voltage method, it is defined such that the full charge is reached if a battery open-circuit voltage reaches 4.2V/cell, in a case of a lithium ion battery, for instance. For a case of a rechargeable battery containing a large number of cells, the full charge is defined as that the battery open-circuit voltage of one of the cells reaches 4.2V. Thus, the voltage method ensures the detection of the full charge by measuring the open-circuit voltage.
In order to detect a voltage value more precisely in the voltage measurement, it is preferable to stop a supply of the current during a measurement of the voltage with connecting no load. However, stopping of the current supply in practice is difficult in most cases because it requires complicated control. Even if the current supply for charging is stopped, a polarized voltage inside the battery may not be stabilized, thereby preventing from estimating the correct open-circuit voltage. In view of the above, there may be used a method in which the charging current and a battery current Imp (Impedance) are measured to detect the charging rate without stopping the supply of the charging current.
The above method assumes that the open-circuit voltage is estimated by subtracting an increased amount in the voltage due to an internal resistance of the battery and the charging current from a measured cell voltage. This enables a stable detection of the full charge.
However, a charging method using the above integrating method may need to accurately know, as one of required condition, the overall charging capacity of a pack is known. Accordingly, it is difficult to detect the full charge correctly if the overall charging capacity decreases due to deterioration or fluctuates with an ambient temperature, and may not be able to estimate correctly.
Further, if an error in the measurement occurred in the course of the charging process, the charging rate may not reach 100% as shown in FIG. 1, resulting in failure to accurately detect the full charge.
In case of that the voltage method is used, it is possible to estimate the open-circuit voltage if the charging current is small. However, as shown in FIG. 2, with increasing charging current value, a battery current Imp fluctuates due to an error in the battery direct current Imp or self-heat generation of the cells by the charging current, thereby preventing from obtain correct open-circuit voltage and resulting in failure to accurately detect the charging rate.
Available methods of detecting the remaining capacity of the secondary battery may include a detection method using the voltage method for detecting the remaining capacity of the secondary battery by measuring the battery voltage, and a detection method using the integrating method for calculating the remaining capacity of the secondary battery by measuring and integrating the voltage and the current.
The remaining capacity detection using the voltage method is of measuring a terminal voltage of the battery cell, and then calculating the remaining capacity based on a correlation between the voltage and the battery capacity (a remaining capacity rate) of the secondary battery, so that in the case of the lithium ion battery, for instance, the battery may be judged to be in a full charge condition if the battery voltage reaches 4.2V/cell, or to be in an over-discharge condition if the battery voltage decreases to 2.4V/cell, thereby making the measurement easy.
On the other hand, the remaining capacity detection using the integrating method may be classified into a current integrating method of measuring the current, and then integrating the measured current for each certain period of time, and a power integrating method of measuring the current and the voltage, then calculating the power output by means of a multiplication of the measured current by the measured voltage, and further integrating the calculated power output for each certain period of time. The above integrating methods are both effective in, after calculating a discharge current or a discharge power output, calculating the remaining capacity of the secondary battery from a ratio of the calculated discharge current or discharge power output to a serviceable current or power output supposed in the battery, thereby providing the stable detection of the remaining capacity without being affected by the voltage fluctuations.
However, the remaining capacity detection using the voltage method gives rise to a disadvantage such that accuracy in the detection of the remaining capacity in an intermediate potential range of the secondary battery is extremely degraded at the time of a discharge. This is because in the case of the lithium ion battery, for instance, the voltage in the intermediate potential range as shown in FIG. 9 is so substantially constant as to create no great voltage difference, resulting in a difficulty to detect the remaining capacity with using the voltage.
Further, the remaining capacity detection using the integrating method also gives rise to another disadvantage such that the accuracy in the detection of the remaining capacity is degraded when the end period of the discharge is reached. This is because the measurement error of the voltage and the current or heat loss causes an error to be so accumulated, together with the integrated current or power, as to cause a large error in the end period of the discharge, leading to a degradation of the accuracy.
In view of the above, another method is employed, in which the charging capacity is measured by using the integrating method in combination with the voltage method. This detecting method provides the detection of the charging capacity by using the integrating method from the start of the charging process until a time near the full charge, and then switches the integrating method to the voltage method when approaching to the full charge, permitting the measurement to be performed in regions where effects of the above methods are respectively maximized.
Further, another method is employed to detect the battery capacity using the integrating method in combination with the voltage method. The voltage method provides a high accuracy in the calculation of the capacity if the current of the secondary battery shows a small value, whereas it fails to obtain the accurate open-circuit voltage due to the fluctuations, and the like of the direct current Imp (Impedance) with the ambient temperature or the Imp inside the battery if the above current shows a large value, thereby preventing from accurately calculating the battery capacity. Further, the integrating method provides the high accuracy in the calculation of the capacity if the current of the secondary battery shows the large value, whereas it causes an increase in the integration error with decreasing current, resulting in the degradation of the accuracy in the calculation of the capacity.
In Pamphlet of International Publication Patent No. 98/056059, there is given a description of a method for performing the detection of the battery capacity using the current integrating method in combination with the voltage method. In consideration of characteristics of a current output that shows a constant value if the battery capacity is in a condition close to an empty and decreases as the battery capacity approaches to near the full charge, the detecting method described in the above prior art suggests to use the voltage method if the current output is smaller than a prescribed current value, and to use the current integrating method if the current output is larger than the prescribed current value. This detecting method may provide an increased accuracy in the calculation of the battery capacity by measuring the battery capacity by means of a switching between the voltage method and the integrating method.
However, there is a case where, in the above detecting method using the integrating method in combination with the voltage method, the switching from the integrating method to the voltage method does not always reach an agreement between the charging capacity (or the charging rate) measured by the integrating method and the charging capacity (or the charging rate) measured by the voltage method, thereby resulting some discontinuity between the measured values when the switching is performed. Further, in case where the switching from the integrating method to the voltage method is performed gradually, a method for forcibly correcting the measured values in order to concatenate the above measured values that are not in agreement with each other, so that it has been also disadvantage such that the accuracy in the measurement of the charging capacity or the charging rate is so degraded as to hinder the charging process.